Sir Menzies Campbell will challenge the Liberal Democrats and force them to take risks, he promised yesterday as he was elected leader with a commanding lead over his rival Chris Huhne.

The votes of "armchair" members were enough to counter the economic spokesman's groundswell of support among activists, allowing the 64-year-old acting leader to secure 29,697 (58%) of the votes ahead of Mr Huhne's 21,628 (42%).

But his promise to modernise the Lib Dems and make them "the party of ideas and innovation" raised the spectre of a shift to the right and is already under challenge as activists attempt to remove a controversial debate on the Royal Mail from the agenda of the spring conference in Harrogate, which begins today.

Gordon Brown's jibe - that the party had been in search of a leader, and now the leader was in search of a party - may have a certain truth. Many of those backing Sir Menzies are from the economically liberal Orange Book wing and want Sir Menzies to push the party into reconsidering its old assumptions on issues such as public service provision.

"Let me make it clear now that caution and consolidation will not do," the new leader warned colleagues who had gathered at Westminster to hear the results. "A safe pair of hands, yes, but ready to take risks, ready to challenge orthodoxy and ready to challenge the party too."

All Lib Dem policy is made at the party's twice yearly conferences. All leaders can do is encourage activists to back change.

Tomorrow's motion - which would call for the sale of almost half the Royal Mail to shareholders - is seen as a move to the right. Critics say it will provoke furious debate, as a similar proposal did at the autumn conference, and should be removed from the agenda so it does not overshadow Sir Menzies' election.

"It's very much a first step for the Orange Bookers," said Lord Greaves, a veteran activist opposing the motion. "There's a suspicion that under Ming's leadership they will be promoted to high places. The word 'modernisation' in British politics means the Blairite agenda."

But Norman Lamb, the trade and industry spokesman, insisted the motion would save thousands of post offices and said the party needed a policy on a service in crisis. Another crucial motion - put forward by activists - would allow the new leader to clear the slate, by axing all policies passed since 1999 unless they were renewed by October next year.

Sir Menzies also focused on core Lib Dem issues yesterday, promising to champion the environment, civil liberties and the decentralisation of power and to lead "a crusade against poverty - the poverty of income and the poverty of aspiration". He promised to make the Lib Dems the party of "democratic revolution", combating an unelected quango state and unaccountable central government.

His predecessor Charles Kennedy, who pledged his full backing, said: "I think the centre of gravity of the party, philosophically and politically, is very clear. I don't think that is going to change much at all."

Mr Huhne, who began the race as rank outsider, has almost as much to celebrate as the new leader. He has laid down a strong marker for a future bid.

"Chris has won the campaign and built a national political base in eight weeks. His turn will come," said his adviser, Lord Oakeshott.

Simon Hughes, the Lib Dem president, came third in the first round of voting, polling just 12,081 votes against Sir Menzies' 23,264 and Mr Huhne's 16,691 on a turnout of 72%. The second preferences of his voters were then distributed to the other candidates.

Mr Huhne said party members had made the right decision, adding: "Ming has undoubted authority, experience and credibility."